Field of Application
The present application relates to an inverter control technology. More particularly, the present application relates to a control signal generating system and an inverter control device thereof.
Description of Related Art
An inverter is a power conversion device which operates based on power electronic technology. By utilizing appropriate control methods, electric power can be converted from direct current (DC) to alternating current (AC) or from AC to DC. Generally speaking, an inverter comprises a switch unit and a filter unit. The switch unit inverts a DC voltage to an AC voltage. The filter unit receives the output of the switch unit and filters the high frequency components therein to generate the required AC voltage. The filter unit then outputs the AC voltage to the AC port. However, for an inverter connected to a motor or transformer winding, the inverter may not comprise the above-mentioned filter unit. Under the circumstances, the AC voltage output by the switch unit is output to the AC port directly.
The inverter topology is usually but not limited to a two-level three-phase bridge circuit or a multi-level inverter circuit. The multi-level inverter circuit may be a three-level neutral point clamping inverter circuit. Additionally, the filter unit may be but not limited to an L filter, an LC filter, or an LCL filter, etc. In this sense, the filter unit may be another more complex filter structure depending on practical requirements. In addition, the inverter may be but not limited to a three-phase or a single-phase system.
A typical inverter operates as follows. First, the DC port of the inverter is connected to a DC power (such as a battery, a super capacitor, or a DC power supply obtained by another distributed power generating unit via a power conversion device). After that, the inverter converts the received DC power supply through the switches in the switch network. The converted DC power supply is then filtered by the filter unit and output to the AC port. Additionally, the AC port of the inverter is connected to local loads and a grid via switches and isolating transformers (optional) to constitute a micro-grid system.
The micro-grid system may be constituted by one-unit inverter or multi-unit inverters connected in parallel. When the inverter is not connected to a grid, the micro-grid system is in an independent operation state. In other words, the micro-grid system is in an off-grid operation state. When the inverter is connected to a grid, the micro-grid system is in an on-grid operation state.
As for the on-grid operation mode, it is usually assumed that the grid is an ideal voltage source which controls the inverter to allow the inverter to be a controlled current source synchronous with the grid voltage according to the prior art. Such an inverter may be called a current injection inverter. However, when the current injection inverters in the grid account for more and more overall capacity, the grid stability is threatened, which causes the grid no longer to be an ideal voltage source. In addition to that, if the grid entry point is located at a distal end of the grid or in a weak grid, an unstable phenomenon will occur when controlling the prior art current injection inverters.
When considering the independent operation mode, the prior art current injection inverter needs to be switched to voltage source control. This approach will increase the complexity of control, and the load voltage will possibly fluctuate or even be interrupted during the switching process, thus seriously affecting the power supply quality.
For the forgoing reasons, there is a need to solve the above-mentioned inconveniences and shortcomings by providing a control signal generating system and an inverter control device thereof, which is also an objective that the relevant industry is eager to achieve.